1. Field of the Invention
The present invention relates to a fiber laser apparatus which uses a semiconductor laser as an excitation light source to obtain blue laser light using a simple configuration, and an optical multi/demultiplexer that takes laser light out of the fiber laser apparatus.
2. Description of the Related Art
Lasers are desired to emit light with a wavelength equal to that of blue light. This is because such lasers are expected to be applicable to various fields such as displays, optical storage devices, optical information processing, and optical sensors. Of these fields, a typical field that requires high optical power per unit area (optical power (optical density)) is optical storage devices. On the other hand, for applications such as displays, oscillation wavelength is important and significantly high power (optical density) is not required.
As a laser apparatus that emits blue light, an apparatus has been proposed which obtains blue laser light by doping a core of an optical fiber with thulium ions Tm3+ and carrying out upconversion excitation.
For example, Jpn. Pat. Appln. KOKAI Publication No. 7-226551 discloses a laser light source apparatus based on an upconversion method.
An optical fiber has a core portion doped with Tm3+ and Tb3+ ions. Laser light of wavelength 640 to 650 nm emitted by a semiconductor laser as a first excitation light source and laser light of wavelength 670 to 810 nm emitted by a semiconductor laser as a second excitation light source are multiplexed by a multiplexer and then input to one end of an optical fiber. The laser lights excite Tm3+ in the optical fiber to emit light of wavelength 455 nm. The emitted light is repeatedly reflected and amplified by an optical resonator formed in the optical fiber and is then output from the other end of the optical fiber as laser light.
The core portion is doped with Tm3+ in order to allow the energy level of electrons in the core portion to increase up to that of laser light of wavelength 455 nm.
As shown in the above conventional example, it is efficient to use two lights with different wavelengths to excite Tm3+. Further, as high optical power (optical density) as possible is desirably used to excite Tm3+. However, in order to take output light out from one facet of the optical fiber, two lights with different wavelengths must be incident on the other facet of the optical fiber as in the case of this example. As a result, a multiplexer is required which synthesizes two lights of different wavelengths. Consequently, an optical axis must be adjusted at more locations, i.e. between the two semiconductor lasers and the multiplexer and between the multiplexer and the optical fiber. Further, both lights require similarly accurate adjustments, thereby increasing parts and manufacture costs. Therefore, disadvantageously, the apparatus may become expensive.
As an inexpensive multiplexer, a waveguide (Y-shaped waveguide) may be designed in which a core is Y-shaped to provide a plurality of entrance portions. In this case, however, incident light exceeds a critical angle at a merging portion, and is partially radiated out from the core. Thus, optical power (optical density) cannot be increased.
Further, high-power semiconductor lasers of wavelength 650 nm (a wavelength with high energy) are still expensive and are not easily available.
Furthermore, a fiber laser that outputs light of wavelength 635 nm is disclosed in, for example, T. Sandrock et. al. xe2x80x9cHigh-power continuous wave upconversion fiber laser at room temperaturexe2x80x9d, Optics letters, vol. 22, No. 11, Jun. 11, 1997. According to this document, high-power laser light of wavelength 635 nm (a wavelength of high energy) is obtained using as an excitation light source a titanium sapphire laser that provides output light of wavelength 850 nm (a wavelength with high energy) and using an optical laser doped with Pr3+ and Yb3+ ions. With the values shown in this report, the power density of excitation light incident on the core, measured at an oscillation threshold value of the fiber laser, can be estimated at about 0.4 MW/cm2.
On the other hand, presently available high-power infrared emission semiconductor lasers typically provide optical power (optical density) of about 1.0 MW/cm2 at a facet of the chip. This value is slightly larger than that of the excitation light power in the above report. Furthermore, a beam from the semiconductor laser has a large spread angle of xc2x110 to 20xc2x0 in the direction of chip thickness and xc2x1several degrees in the direction of chip width. Thus, it is virtually impossible to further improve optical power (optical density) while maintaining incidence efficiency even with condensation carried out by an optical system. Further, optical power (optical density) cannot be increased even by using a Y-shaped waveguide and a plurality of semiconductor lasers, as described above. Thus, disadvantageously, semiconductor lasers provide low optical power (optical density).
That is, in the prior art, as described above, two semiconductor lasers are used, and lights of different wavelengths from these lasers are incident on one facet of an optical fiber doped with Tm3+. This conventional method requires a multiplexer and also requires that the optical axis be adjusted at more locations. Accordingly, both lights require similarly accurate adjustments, thereby increasing parts and manufacture costs. Further, disadvantageously, optical power (optical density) cannot be increased by the method of using a Y-shaped waveguide to multiplex two lights from different lasers.
It is an object of the present invention to provide a fiber laser apparatus which uses two excitation light sources emitting lights of different wavelengths, to obtain a light beam of a desired wavelength (color) on the basis of light resonance effected using the excitation light sources as well as an optical multi/demultiplexer and an image display device therefor.
To attain this object, according to an aspect of the invention, there is provided a fiber laser apparatus comprising a first excitation light source, a first optical fiber on which light from the first excitation light source is incident through one facet thereof, in which a core is doped with a first rare earth substance, and in which light resonance occurs in the core, the first excitation light source having a resonant section formed therein to output light of a selected wavelength from the other facet thereof, the light of the selected wavelength being included in light of a resonant wavelength; an optical multi/demultiplexer arranged at the other facet of the first optical fiber to reflect and output the light of the selected wavelength in a direction different from that of the first optical fiber; a second excitation light source which supplies light to the resonant section of the first optical fiber via the multi/demultiplexer and the other facet of the first optical fiber; and a second optical fiber which guides the light of the selected wavelength from the optical multi/demultiplexer to an exterior.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.